Generally, the iris authentication identifies a person by illuminating the eye of a user or its circumference with a near infrared ray or the like, by taking the eye image and the circumference image (as will be generally called the “eye image”) with a camera, by extracting the iris information from the eye image obtained, and by comparing the iris information with the iris information which has already been registered in the iris information database. The eye image taking device to be used has to take the eye image highly precisely for extracting the iris information of the user precisely. Thus, there has been proposed an eye image taking device, which utilizes the automatic focusing technique to detect the eye position from the taken image of the entire face and to take the eye image of a desired size by zooming up that eye with a zoom lens.
In this automatic focusing technique, the control is made by making use of the fact that many high-frequency components are contained in image signals if an object is focused (as will be called the “focusing”) so that the contour image is clearly taken, by integrating the high-frequency components in the image signals while varying the lens position, and by searching the lens position, at which the integrated value is the maximum. In order to search the lens position promptly, moreover, there is generally adopted (as referred to JP-A-2000-131598, for example) the so-called “hill-climbing method”, in which the lens position is gradually varied in the direction for the high-frequency components to increase.
In recent years, on the other hand, as the accounts settling system or the like using a mobile terminal device such as a mobile telephone spreads, it has been tried to install the iris authenticating function of a high personal authentication reliability in the mobile terminal device. However, the eye image taking device needs a drive system for driving the lens and has a difficulty in reducing the size and weight of the optical system. It has, therefore, been extremely difficult to install the eye image taking device having the automatic focusing mechanism in the mobile terminal device such as the mobile telephone. For these reasons, a practical eye image taking device for the mobile terminal device can be proposed by installing a small, light and inexpensive camera using a fixed focus lens, by deciding the focusing degree with the magnitude of the high-frequency components contained in the image signals, and by guiding the imaging distance so that the magnitude of the high-frequency components may be larger than a predetermined threshold value.
However, personal differences are large not only in the pattern of an iris and the shapes of the individual portions such as eyelashes or eyelids but also in the magnitude of the high-frequency components contained in the eye image signals. Therefore, the method for guiding the imaging distance so that the magnitude of the high-frequency components may become larger than the predetermined threshold value has a problem that the stable focus decision cannot be attained depending upon the shapes of the individual portions of the eye of the user. This problem is exemplified such that some user may fetch the eye image out of focus or can neither attain the focus decision nor fetch the eye image.
The present invention contemplates to solve the problems thus far described and has an object to provide an eye image taking device, which can make a stable focus decision irrespective of the shapes of the individual portions of the eye of a user by using the small, light and inexpensive fixed focus lens and which can be installed in a mobile terminal device.